In recent years, secondary batteries represented by a lithium ion battery have increasingly wide range of applications, mainly for in-vehicle use, and the production volume is constantly increasing. In the process of manufacturing a secondary battery, a method is often employed, in which the components of the battery are sealed in a metal case. In addition, in order to improve the space efficiency of the secondary battery, a rectangular cell case is often employed rather than a conventional circular case. In both cases where the circular cell case is employed and where the rectangular cell case is employed, laser welding, by which a structural body can be manufactured efficiently, is more often used for manufacturing lithium ion batteries.
In the manufacturing process of a lithium ion battery, laser welding is often used in the following three processes:                1. Cap seam welding for bonding an aluminum can body and a cap member.        2. Seal welding for closing a pouring hole for pouring an electrolyte.        3. Component welding for electrically connecting a plurality of cells in parallel or in series.        
However, the seal welding of the second process among the above-mentioned processes is known to be technically difficult because the surroundings of the pouring hole, which tend to be easily contaminated with an electrolyte, need to be welded by a laser. When a metal lid for sealing is placed and welded on the pouring hole with some adhering electrolyte thereon, a weld defect occurs due to the influence an electrolyte or a solvent contained in the electrolyte. Even when welding is performed after removing the adhering electrolyte by suction or wiping, a bonding defect possibly due to the electrolyte will occur at a certain rate.